A tightening device of this type has already been proposed which comprises an epicyclic train for giving low-torque high-speed rotation when rotated in its entirety and for delivering high-torque low-speed rotation from a planet gear support frame when the inner gear of the train is brought out of rotation (Published Examined Japanese Patent Application SHO No. 57-48348).
The tightening device includes a drive shaft which serves also as a sun gear and with which the inner gear is engageable by a clutch. The clutch is adapted to automatically disengage when the torque acting on an output shaft exceeds a specified value.
However, when the load on the output shaft decreases during high-torque low-speed rotation, the clutch automatically engages to alternatively bring the output shaft into high-speed rotation again, hence hazardous.